Improved techniques for stabilization of rabbit-mouse hybridomas have allowed the preparation of a number of new cell lines. Although all of the cell lines which were prepared in the past secrete either an immunoglobulin heavy or light chain, but not both, two of the most recently prepared cell lines appear to secrete both heavy and light chains from the rabbit. Characterization of these and other hybridoma products is well underway. These studies have revealed several unusual serological and structural characteristics of the secreted products. In the course of the fusion experiment directed toward preparation of rabbit-mouse hybridomas it was observed that rabbit spleen cells, when in culture, give rise to a large cell which has a long life in culture. Characterization of these cells indicated that they were nonadherent macrophages. In collaborative studies with scientists at the Institut Pasteur in Paris, structural characterization is being undertaken for allotypes which are found only in wild rabbit populations. The serologic characterization of the light chains indicate that they have allotypic markers which are similar to but not identical with those from the domestic rabbit population.